1. Field of the Invention
This invention relates to an optical fibre bend sensor.
2. Discussion of Prior Art
A deformation of an optical fibre results in strain developing within the fibre. Strains can be categorised depending on the nature of the distortion which produces them. A longitudinal strain: a one-dimensional expansion or compression of the fibre along its length, is categorised as a scalar strain. A scalar strain by its nature requires only one parameter, the magnitude of strain along the axis of the stretch or compression, to characterise it. In terms of the strain tensor (xcex5) this longitudinal strain is the tensile component xcex5zz.
If however a fibre is also free to deform in a transverse plane i.e. that perpendicular to the length of the fibre, further components of the general strain tensor will have effect. If the fibre is restrained in its position, tensile strain (xcex5xx, xcex5yy) arises within a compressed or stretched fibre in perpendicular directions to the longitudinal strain described above. This is known as transverse strain.
More generally, if the fibre is not fixed rigidly in location, one end of the fibre may then be displaced with respect to the other. Such a displacement occurs in three dimensions and results in a xe2x80x9cbendingxe2x80x9d of the fibre. Both this bend and three-dimensional tensile strain require characterisation by both magnitude and direction. In the simple case of a bend with negligible fibre elongation, the induced strain is conveniently described in terms of the bend magnitude and plane of curvature. This curvature (xcexa) has magnitude equal to 1/R, where R is the radius of curvature, and direction defined by the normal vector pointing towards the centre of curvature. Assuming a linear strain gradient across the bend then, in mathematical terms, the deformation developed in bending is formally equivalent to the transverse strain gradient: ∇txcex5zz, where ∇t is the transverse gradient operator.
Optical fibre sensors for strain measurement are known in the prior art. An optical fibre is embedded in or surface-bonded to the structure to be monitored and its optical properties observed. The monitored structure is not limited to engineering applications e.g. aeroplane structures, building walls; optical strain sensors have been found useful in the medical field. A number of external influences may cause strain to develop within a structure: applied stress (elasticity) and electric field (piezoelectricity) to name two. An optical fibre within such a strained structure will in turn experience the effects of such strains. Transverse strain components will affect the refractive index and longitudinal strain components will also stretch (or compress) the fibre. In either case the optical path length of radiation propagating within the fibre is changed. Thus information pertaining to strain within the monitored structure is manifest in the phase of radiation propagating within the fibre and is therefore extractable using interferometric techniques. Optical strain sensing is particularly attractive because interferometry offers an accurate detection tool and the sensitivity of optical properties to physical influences such as strain and temperature is high.
Temperature has a similar effect to strain on an optical fibre. Thermal expansion will change the length and refractive index of the fibre and additional strains may also be induced by the differential expansion of fibre and host material. Any optical technique purporting to measure strain must make allowance for this cross-sensitivity between temperature and strain.
A problem with prior art interferometric strain sensing techniques such as that described in patent application GB9606785.5 is that a single probe fibre measures only a scalar component of strainxe2x80x94the elongation of the fibre length. Transverse strain components are not measured and a general three dimensional contortion of a fibre is detectable only as a change in fibre length. Fibres have been multiplexed both in parallel and series in order to provide scalar strain measurements across a range of positions. From data gathered from an array of such single probes a map of strain gradient may be built up. However each probe intrusion inevitably weakens the structure being monitored. The interface region will be subject to increased strain and there is a clear benefit to be had in limiting the number of such interfaces.
Strain gradient measurements have been performed from sea-going vessels using a number of magnetic bearing sensors interspersed with depth sensors on a towed sonar array. However such arrays are very bulky, occupying considerable storage space when not in use and such considerations limit the practical length of the array.
Furthermore magnetic bearing sensors as currently used are affected by perturbations of the local magnetic field. Measurements are therefore influenced by metallic structures on the sea bed as the array passes above.
An optical fibre sensing device sensitive to the degree of fibre bending is disclosed in U.S. Pat. No. 4,443,698. The interference pattern developed between light propagating in two different cores of a multicore fibre is used to monitor changes in the bending of the fibre. If two cores are used then a helical 90xc2x0-twist about each other is incorporated over the sensing region. This allows the fibre to be sensitive to bending regardless of bend plane, but removes any capability of measuring bend direction. With three or more non-coplanar cores the need for such a twist can be avoided. The invention employs phase tracking techniques to follow varying bend parameters in order to allow deduction of unambiguous measurements. However, tracking requires access to one of the interferometer optical paths, a clear disadvantage for applications that require remote addressing of passive sensing lengths. In this case bend direction information will be lost. Tracking will also be lost if power is not continuously maintained.
There is a perceived need for non-intrusive shape sensing by means of bend measurement. Such a sensor would have many applications in diverse fields. In robotics, knowledge of the absolute position of moving parts is essential. This can be deduced if directional bending of an integrated fibre can be measured. In medical applications, any internal monitoring is safest with minimal intrusion from a foreign probe and, additionally, negligible generation of external electromagnetic fields. Lightweight position monitoring is essential to promote mobility in an artificial limb. Prior art optical strain sensors do not measure strain gradients and prior art shape sensors are overly bulky and generally rely on magnetic effects which have neither the accuracy nor immunity from environmental perturbation afforded by optical measurement.
It is an object of this invention to provide a bend sensor capable of monitoring both bend magnitude and orientation and which is constructable in compact form.
The present invention provides a bend sensor incorporating a fibre assembly arranged to receive incident radiation and analysing means for analysing radiation output from the fibre assembly characterised in that the fibre assembly comprises: a multicored optical fibre having component cores which are sufficiently separated to counteract crosstalk; coupling means for coupling radiation propagating in a first core into a second core of the fibre; and reflecting means arranged to reflect a portion of incident radiation; wherein the reflecting means and coupling means are arranged to define first and second optical paths within the fibre assembly such that the paths have an optical path difference developed over a sensor length section of the fibre within which the first optical path corresponds to radiation propagation in the first core and the second optical path corresponds to radiation propagation in the second core; and the analysing means is arranged to disperse interferograms formed between radiation propagating in the first and second optical paths.
This invention provides the advantages of accuracy and relatively non-intrusive bend measurement. Optical fibre assemblies can be produced with very small diameter and embedding within a structure will thus result in minimal disruption of that structure. The apparatus is capable of determining bending of the sensor length, perhaps as a consequence of strain within an embedding structure, by monitoring that component of the bend in the plane of two fibre cores within the sensor length. Interferograms are formed between radiation propagating along two different optical paths, the optical paths differing within a specific region of the fibre. This region, the sensor length, may be only a fraction of the total fibre length. Generally, bending this sensing region will inevitably lengthen one core with respect to the other. Interrogation of this length differential by means of interferometry provides an accurate tool with which to measure bending. Moreover, defining a sensor length down a potentially long fibre downlead enables strains to be detected at a localised region remote from the radiation input end of the fibre. Thus the fibre assembly can be incorporated in, for example, a building wall, and strains developing in the deep interior of the wall measured.
The first and second cores constitute a core pair and component cores of the multicore fibre preferably comprise an arrangement of such core pairs; and the coupling means may accordingly be arranged to couple and reflect a portion of radiation propagating in the first core into the second core of the respective core pair.
This provides the advantage of flexibility. The optical path difference arising between any core pair can be interrogated, and this embodiment therefore provides a selection of planes each of which is capable of being the plane in which components of a general bend curvature are measured.
The sensor may also include signal processing means arranged to extract interferogram phase variation from the dispersed interferograms and to calculate bend curvature (xcexa) of the sensor length from the phase variation. This provides the advantages of speed and accuracy generally to be had from signal processing power in extracting a useful parameter, bend curvature xcexa, from interferometric data.
In a preferred embodiment, respective interferograms are generated from radiation propagating within each core pair, each interferogram arising from differences in respective first and second optical path lengths; and the resultant calculated bend curvature (xcexa) is that component of bend curvature (xcexax, xcexay) in a plane containing the sensor length respective core pair. Moreover, the multicore fibre preferably comprises at least two core pairs contained in non-coincident planes, thereby enabling calculation of absolute bend magnitude and orientation from corresponding in-plane components of bend curvature.
Such use of multiple core pairs enables bend plane to be advantageously determined. A single core pair can be used to obtain a measurement only of a general three dimensional curvature projected onto a specific plane: that of the core pair. However measurement of two non-coplanar projections, by means of two non-coplanar core pairs, enables both the degree of curvature and the orientation of the plane of the curve to be deduced. Preferably, the two non-coplanar core pairs are contained in orthogonal planes. This maximises the likelihood of an accurate measurement of any bend direction within a general three-dimensional volume. If more than two core pairs are used to measure multiple projections of a single curvature, magnitude and orientation determination can be made with increased accuracy.
It is preferred that the cores within each pair have unequal effective refractive indices. This enables bend xe2x80x9chandednessxe2x80x9d to be determined in cases for which it is ambiguous. Although bend magnitude can be determined using a single core pair of equal refractive index, such an arrangement cannot distinguish between a bend to the right and a bend to the left i.e. there is no indication of which particular core of the pair is compressed. However, by arranging for the cores to be distinguishable in terms of optical path length, a means for determining which of the pair is compressed is provided. The optical path difference measured by this embodiment of the invention is now dependent on the direction of the bend.
There are a number of known ways of producing unequal core effective refractive indices. For example, they may be produced during manufacture of the fibre, i.e. by using slightly different levels of doping per core, or after manufacture by injecting high intensity UV-radiation into one of the cores for a short time. The latter technique has the effect of slightly increasing the refractive index of the core in a similar way to Bragg grating manufacture, and also offers more flexibility.
The multicored fibre is preferably a bunched multiple monomode (BMM) fibre. Such fibres are known and comprise multiple fibre cores, each with an associated cladding xe2x80x9cregionxe2x80x9d. Each cladding region is smaller in cross-sectional area than would be required for typical cladding of cylindrical symmetry. This enables the cores to be more closely spaced than previously permitted, with regard to the requirements for avoiding crosstalk. This in turn results in an overall reduction of the diameter of a multicore cable. This is advantageous to many applications of bend sensing in which it is desirable to minimise the disruption to a structure under observation by intrusion of a probe.
The component cores may be stress-induced highly birefringent (HiBi) cores. This provides the sensor with the capability of discriminating between the effects of temperature and strain.
Alternatively, the multicored fibre may be a photonic crystal fibre. This again provides the advantage of compactness. A photonic crystal fibre is another example of a multicored fibre in which crosstalk can be kept to an acceptable level, but overall fibre diameter is reduced in relation to a traditionally structured fibre bundle.
In a preferred embodiment the fibre assembly is arranged to receive incident broadband radiation and the analysing means is arranged to form a channelled spectrum by dispersing interferograms formed between radiation propagating in the first and second optical paths as a function of wavelength. Suitable broadband radiation may be provided by a superluminescent diode, an Erbium Doped Fibre Amplifier or a Praseodymium light source. These sources do not all operate over the same wavelength range and appropriate detectors need to be used in each case. Interferometric techniques rely on measuring changes across the interference pattern as a function of the phase of the radiation forming it. It is therefore necessary to scan across a range of phase angles. Commonly optical path difference is scanned, although an equivalent phase scan may be achieved by scanning radiation wavelength. Thus, whereas a Michelson interferometer scans optical path difference at constant wavelength, this embodiment of the invention scans radiation wavelength at constant optical path difference. The principal advantage of this implementation is that it requires neither an external reference path nor any motion of the sensing length, which would be impracticable in many applications, to compensate for the optical path difference introduced by the bend. This reduces the ancillary apparatus required to interrogate the sensing fibre once it has been embedded in a structure.
Alternatively the sensor may also include a scanning interferometer arranged to perform a scan of optical path lengths defined within and whereby interferometer optical path differences compensate for the optical path difference between first and second optical paths and the analysing means is arranged to disperse interferograms formed between radiation propagating in the first and second optical paths (122, 124) as a function the interferometer scan. The interferometer may be arranged to perform either a spatial or temporal scan.
The component cores may be distributed in a symmetrical arrangement within the multicore fibre. The fibre is preferably less than 130 xcexcm in diameter.
The coupling means is preferably a tandem coupler. A tandem coupler is an optical element which reflects a proportion of the radiation incident on it from one direction, transmits another proportion and at the same time couples the reflected radiation into another fibre core. These optical properties make the device eminently suitable for defining an optical path which encompasses two fibre cores, such as is required for embodiments of this invention in which the sensing fibre length is addressed via a downlead in which the cores define common beam paths.
The tandem coupler preferably incorporates a beamsplitter wherein the beamsplitter comprises a base and, non-coplanar with this, respective pairs of planar surface faces for each core pair wherein each planar surface face extends from the base towards an apex and faces within each pair are mutually disposed at approximate right angles. Such a beamsplitter conveniently provides in a single component the reflection and transmission characteristics required for the functioning of the tandem coupler.
The multicored fibre may have four symmetrically oriented component cores and the beamsplitter may comprise a square base, four triangular surface faces and an apex wherein each triangular face extends from one side of the square base to the apex and the perpendicular distance from the apex to the base is one half of the distance of one side of the base. Such a symmetric arrangement of cores means that the same accuracy of measurement can be had regardless of bend plane orientation.
Within the tandem coupler the beamsplitter may be located intermediate to first and second planar microlens arrays wherein each microlens array includes a respective microlens located a focal distance away from each component core and wherein the microlenses of each array are arranged in a common plane and the common planes of each lens array are mutually parallel and parallel to the base of the beamsplitter. The microlenses in these arrays thus either produce a collimated beam from radiation exiting the cores, or couple collimated radiation into the fibre cores. This aids operation of the tandem coupler.
A further aspect of the invention provides a temperature sensor incorporating a fibre assembly arranged to receive incident radiation and analysing means for analysing radiation output from the fibre assembly characterised in that the fibre assembly comprises: a multicored optical fibre having component cores which are sufficiently separated to counteract crosstalk; coupling means for coupling radiation propagating in a first core into a second core of the fibre; and reflecting means arranged to reflect a portion of incident radiation; wherein the reflecting means and coupling means are arranged to define first and second optical paths within the fibre assembly such that the paths have an optical path difference developed over a sensor length section of the fibre within which the first optical path corresponds to radiation propagation in the first core and the second optical path corresponds to radiation propagation in the second core; and the analysing means is arranged to disperse interferograms formed between radiation propagating in the first and second optical paths.
In another aspect, this invention provides a beamsplitter comprising a base and, non-coplanar with this, respective pairs of planar surface faces wherein each planar surface face extends from the base towards an apex and faces within each pair are mutually disposed at approximate right angles. Such a beamsplitter conveniently provides reflection and transmission characteristics which are appropriate to various optical applications.
The beamsplitter may be for reflecting a component of radiation propagating in a first core of a multicored fibre into a second core of the fibre, first and second cores comprising an opposite core pair, wherein each opposite core pair is aligned with a respective pair of beamsplitter planar surface faces such that an optical path exists from the first core to the second core via reflections from the planar surface faces.
Preferably, the multicored fibre has four symmetrically oriented component cores and the beamsplitter has a square base, four triangular surface faces and an apex wherein each triangular face extends from one side of the square base to the apex and the perpendicular distance from the apex to the base is one half of the distance of one side of the base.
A tandem coupler may incorporate a beamsplitter in accordance with the above description located intermediate to first and second planar microlens arrays wherein each microlens array includes a respective microlens located a focal distance away from each component core and wherein the microlenses of each array are arranged in a common plane and the common planes of each lens array are mutually parallel and parallel to the base of the beamsplitter. Such a tandem coupler has many applications in devices in which it is required to define an optical path encompassing two fibre cores.